Ceramic mass and a capacitor having the ceramic mass

ABSTRACT

A ceramic mass has a phase-heterogeneous ceramic of m weight percent of a phase composed of BaNd 2 Ti 4 O 12  with negative temperature coefficient of the dielectric constant and 100-m weight percent of a phase composed of Nd 2 Ti 2 O 7  with positive temperature coefficient of the dielectric constant, with m having a value of 50&lt;m&lt;70, and an additive of glass frit that contains ZnO, B 2 O 3  and SiO 2  and whose weight amounts to between 3 and 10 weight percent of the ceramic. The ceramic mass is used to construct a multi-layer capacitor with copper electrodes.

BACKGROUND OF THE INVENTION

The invention is directed to a ceramic mass having a combination of aceramic and a glass frit and to a capacitor having this ceramic mass.

Ceramic masses are known that are employed as a dielectric formulti-layer capacitors with metal electrodes. For cost reasons, copperis preferred as an electrode material. Given the employment of copper asthe electrode material, however, it is necessary to reduce the sinteringtemperature of the ceramic mass below the melting temperature of thecopper since the multi-layer capacitors are manufactured by a commonsintering of the ceramic with the electrodes.

Technical solutions have already been disclosed that allow the commonsintering of a ceramic mass with the Cu electrodes under reducingconditions, whereby the sintering temperature is lowered below themelting temperature of the copper (1083° C.). Specific sintering aids,preferably additives of the glass frits whose material basis is a systemcontaining lead oxide and/or bismuth oxide, are utilized for thispurpose. An oxygen partial pressure <10⁻² Pa must be employed in orderto suppress the oxidation of the copper during the sintering in theregion of 1000° C. At the same time, a critical lower limit of theoxygen partial pressure dare not be downwardly transgressed sinceotherwise the ceramic or a constituent of the glass frit added in themanufacture is subjected to a reduction, which necessarily leads to alowering of the insulation resistance and an inadmissible increase ofthe dielectric losses. In order to avoid a local downward transgressionof this critical lower limit, the decarbonization of the green memberemployed in the manufacture must have been completely realized beforethe beginning of the sintering.

EP 0 534 802 A1, U.S. Pat. No. 5,479,140, U.S. Pat. No. 5,493,262, U.S.Pat. No. 5,488,019, U.S. Pat. No. 5,485,132 disclose ceramic masses ofthe material systems BaO—TiO₂-(RE)₂O₃ wherein the oxide of the rareearth metals (RE) can be partially replaced by Bi₂O₃ and the sinteringcompression thereof already partially succeeding at 900° C. in thatglass frit parts that contain CdO, PbO or Bi₂O₃ or glasses of the systemZnO—B₂O₃—SiO₂ are added. This enables a common sintering with Agelectrodes in air. Compared to a partial reduction that results in alowering of the insulation resistance and an increase in the dielectriclosses, the systems prove insufficiently stable for a common sinteringwith the copper electrodes under inert conditions, for example in anitrogen atmosphere.

EP 0 534 801 A1, U.S. Pat. No. 5,458,981 and U.S. Pat. No. 5,292,694likewise disclose BaO—TiO₂—SE₂O₃ ceramic masses in conjunction with theglass additives containing B₂O₃ and ZnO for the purpose of the commonsintering with silver electrodes. In these instances, too, thedecarbonization upon air admission prevents the combination with copperelectrodes, so that recourse must be had to silver or silver/palladiumalloys. The advantage of a cost-beneficial employment of the silverelectrodes is opposed by the disadvantage of the high mobility of thesilver, particularly at high temperature, that can lead to the migrationeffects and a deterioration of the dielectric properties resultingtherefrom.

According to DE 197 49 858, the materials system BaO—PbO—Nd₂O₃—TiO₂ usedfor the manufacture of the COG capacitors and microwave resonators witha high dielectric constant (DK) is tapped in the region of the phaseformation of rhombic bronzes (Ba_(1−y)Pb_(y))_(6−x)Nd_(8+2x/) 3Ti18O54with 0.6 <x<2.1 and 0<y<0.6 for a sintering at temperatures <1030° C.and, thus, for the common sintering with the Cu electrodes in that thesintering aids, preferably PbO-free glass frits having a specificcomposition are added and a complete decarbonization in nitrogen isachieved due to the action of a water steam at an elevated temperatureupon utilization of the steam reforming process known from crude oilprocessing. It must be noted as a limitation on this technical solutionthat the stability of the ceramic is limited by the PbO content, whichrequires an especially careful decarbonization and an extremely carefulavoidance of too low an oxygen partial pressure. The two demands arelinked to one another since, in particular, the inadmissible downwardtransgression of the critical oxygen partial pressure limit caused bythe slight organic residual constituents must also be locally avoided.Otherwise, a eutectic Pb/Cu alloy is formed that melts at 954° C., whichleads to an electrode run out.

Systems free of PbO and Bi₂O₃ have proven suited for avoiding such adisadvantage. DE 198 41 487 A1 disclosed the PbO-free materials systemBaO—Nd₂O₃—Sm₂O₃—TiO₂ in the region of the phase formation of the rhombicbronzes Ba_(6−x)(Sm_(y)Nd_(1−y))_(8+2x/3)Ti₁₈O₅₄ for the manufacture ofthe COG capacitors and microwave resonators, whereby a temperaturecoefficient of the capacitance TKC<30 ppm/K or, respectively, atemperature coefficient of the resonant frequency TKν₀<10 ppm/K isdesignationally set by means of a suitable selection of the compositionparameters x and y, and a sintering at temperatures <1030° C. and, thus,a common sintering with the Cu electrodes is simultaneously achieved inthat a glass frit with a suitable composition is added in an appropriateamount. The use of this advantage assumes that the rhombic bronzes ofthe appertaining composition are completely formed as a uniform phasebefore the sintering, which makes the relatively high conversiontemperature of 1250° C. necessary in the calcination of the mixture ofthe oxide raw materials BaCO₃, Nd₂O₃ and TiO₂.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to offer a ceramic masssuitable for capacitors that can be manufactured from the oxide rawmaterials by calcination at a temperature of at the most 1240° C.Further, the ceramic mass should be capable of being sintered at atemperature <1030° C. and should exhibit a low temperature coefficientof the dielectric constant.

This object is inventively achieved by a ceramic mass that contains aphase-heterogeneous ceramic and an additive admixture of glass frit. Thephase-heterogeneous ceramic comprises m weight percent of a first phaseand 100-m weight percent of a second phase. The first phase is composedof BaNd₂Ti₄O₁₂ with a negative temperature coefficient of the dielectricconstant, whereas the second phase is composed of Nd₂Ti₂O₇ with apositive temperature coefficient of the dielectric constant. 50<m<70thereby applies to the mixing parameter m. The glass frit contains zincoxide, boron oxide and silicon oxide and has a weight that amounts tobetween 3 and 10 weight percent of the phase-heterogeneous ceramic.

In addition to the constituents, the ceramic mass can also containslight amounts of other, standard constituents that do not deterioratethe desired properties of the ceramic.

The inventive ceramic mass has the advantage that thephase-heterogeneous ceramic, which is contained in it, can already bemanufactured from the raw materials barium carbonate, neodymium oxideand titanium oxide by calcination at a temperature of 1180° C. Due tothe comparatively low calcination temperature, the inventive ceramicmass can be manufactured with a low heat expenditure. The inventiveceramic mass has the further advantage that it exhibits a highdielectric constant ε>50.

Over and above this, the inventive ceramic mass has the advantage thatit can be sintered at temperatures <1030° C., as a result whereof thecommon sintering with the copper electrodes becomes possible.

In an especially advantageous embodiment of the invention, the glassfrit in the ceramic mass comprises the following composition:(ZnO)_(58.5)(B₂O₃)_(31.45)(SiO₂)_(10.05.)

The invention also specifies a capacitor wherein the inventive ceramicmass is employed as the dielectric. The dielectric forms a base bodythat comprises a respective contact layer at two opposite sides. Thecontact layers are contacted to electrodes that are located in theinside of the base body and inter-engage comb-like. Since the ceramicmass employed in the capacitor contains neither lead oxide nor bismuthoxide, the ceramic mass is especially stable with respect to theinfluences with a reducing effect, so that the capacitor has theadvantage of a stable capacitance over the long-term.

In an especially advantageous embodiment of the capacitor, theelectrodes are composed of copper and are sintered together with theceramic mass. Copper has the advantage that it is inexpensive to acquireand exhibits high conductivity. As a result thereof, the capacitor canbe advantageously employed in the range of high frequencies.

The inventive capacitor can be especially advantageously configured inthat the composition of the ceramic is set such by means of a suitableselection of the parameter m that the temperature coefficient of thecapacitor capacitance meets the demands of what are referred to as the“COG characteristic”. The “COG characteristic” means that thetemperature coefficient of the capacitance ΔC/ΔT of a COG capacitor isless than 30 ppm/Kelvin in the temperature interval between −55° C. and125° C. Since the temperature coefficient of the capacitor capacitanceis essentially dependent on the temperature coefficient of thedielectric constant of the ceramic mass employed, a minimization of thetemperature coefficient of the capacitor capacitance can be achieved bymeans of a suitable compensation of the temperature coefficient of theindividual phases of the ceramic.

In the material system BaO—Nd₂O₃—TiO₂, the compound BaNd₂Ti₄O₁₂, whosetemperature coefficient of the dielectric constant TKε₁ amounts toapproximately −120 ppm/K, can be combined with the compound Nd₂Ti₂O₇,which exhibits a temperature coefficient of the dielectric constant TKε₂of approximately +200 ppm/K. The combination is based on the mixing ruleTKε=ν₁ TKε₁+ν₂ TKε₂ to form a heterogeneous phase mixture that yields atemperature coefficient of the capacitance TKC=TKε+α_(L) close to zerofor the capacitor manufactured therewith. The symbol ν indicates thevolume percent content of the constituents and α_(L) indicates the valueof the coefficient of thermal expansion of the ceramic mass.

The sintering at temperatures <1030° C., which enables a commonsintering with the Cu electrodes, is tapped given employment of an inertgas atmosphere with adequately low oxygen partial pressure in that theceramic has a glass frit of the composition added to it as the sinteringaid.

It is also advantageous that, due to the knowledge of thecomposition-dependency of the TKC values, a shift of the temperaturecoefficient TKε toward either positive or negative values conditioned bythe admixture of glass frit can be compensated with a designationalmodification of the composition (selection of the parameter value m).

One advantage of the invention is that the phase-heterogeneous mixtureof the raw materials BaCO₃, Nd₂O₃, TiO₂, which is composed ofBaNd₂Ti₄O₁₂ and Nd₂Ti₂O₇, is already accessible to the calcination at aconversion temperature of 1180° C. and that the sinter compression afterthe addition of a part of a glass frit can be implemented at 930° C. upto a maximum of 1030° C. in the presence of the Cu electrodes under anoxygen partial pressure <10⁻² Pa without the properties typical for theCOG capacitors experiencing any deterioration as a consequence of apartial reduction.

The complete decarbonization of the green member produced during thecourse of the manufacturing process succeeds in a temperature rangebelow the commencement of the sinter compression in that the processknown from petrochemicals for decomposing hydrocarbons or, respectively,organic compounds, which are derived therefrom, into carbon dioxide andhydrogen under the influence of water steam at the elevated temperature(“steam cracking”) is transferred onto the ceramic process. For example,a slightly negative free enthalpy can be estimated from thethermodynamic data for the decomposition of polyethylene glycol orpolyacrylic acid as a binder according to the reaction

so that the procedure of decarbonization of the green members that mustbe undertaken in order to avoid an oxidation of the copper undernitrogen (oxygen partial pressure <10⁻² Pa) can sequence completely.

The invention is explained in greater detail below on the basis ofexemplary embodiments and the FIGURE.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows an inventive capacitor in a partially cut perspectiveview by way of example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows an inventive capacitor with a base body 1 thatcomprises a respective contact layer 2, 3 at two opposite sides. Thecontact layers 2, 3 can be manufactured from a copper stoving paste. Thebase body 1 is composed of the inventive ceramic mass and forms thedielectric of the capacitor. Electrodes 4 that are advantageouslycomposed of the copper and engage comb-like in one another are arrangedinside the base body 1. The inventive ceramic mass is of such a naturethat the capacitor can be manufactured by sintering on the basis of thecommon sintering of the ceramic mass with the copper electrodes.

The manufacture of the inventive ceramic mass and of the ceramiccontained in the ceramic mass is described below on the basis of variousexemplary embodiments:

By mixing the powdered initial substances BaCO₃, Nd₂O₃ and TiO₂ in thecorresponding mol ratio and converting the mixture at 1180° C., thecompounds BaNd₂Ti₄O₁₂ and Nd₂Ti₂O₇ are obtained as a mix wherein thenegative temperature coefficient of the dielectric constant of the firstcompound and the positive temperature coefficient of the second compoundsupplement one another to a value close to zero based on the mixing ruleTKε=ν₁TKε₁+ν₂ TKε₂ with ν₁, ν₂ as the volume percentages of the twoconstituents. Due to the relationship TKε=−2 TKν₀−2 α_(L), a setting ofthe temperature coefficient of the microwave resonant frequency TKν₀ toa value close to zero can likewise be achieved with a suitable selectionof the composition, which enables an employment as the microwaveresonator ceramic.

In order to check the validity of the mixing rule, the melange obtainedafter the conversion is first subject to a grinding process without theglass frit additive (average grain size about 0.6 μm). The fine-grainedpowder mixture that is obtained is subsequently converted into granulesand the latter is compressed by pressing into disk-shaped specimens orcylindrical bodies suitable for resonance measurements and these aresubsequently sintered for 6 hours at 1350 through 1380° C. to between 95through 97% of the theoretically possible, maximum density. After such aprocedure, for example, disk-shaped specimens (S) having a diameter of12 mm and a height of 0.6 mm or, respectively, cylindrical specimens (Z)having a diameter of 10 mm and a height of 6.4 mm are obtained.

By applying electrodes, the disk-shaped specimens are suited formeasuring the dielectric constant ε, the loss angle tan δ at a frequencyof 1 MHz and the temperature coefficient TKC of the capacitance of acapacitor formed with the ceramic. The measured values are recited inTable 1. The cylindrical specimens are suited for measuring the qualityfactor Qν₀ and the temperature coefficient of the resonant frequencyTKν₀ of a microwave resonator formed from them, both of these beingrecited in Table 1.

Table 1 recites the ceramic specimens (S) and (Z) obtained by sinteringat high temperature and having the following compositions:54.8 m-% BaNd₂Ti₄O₁₂/45.2m-% Nd₂Ti₂O₇  (1)61.4m-% BaNd₂Ti₄O₁₂/38.6m-% Nd₂Ti₂O₇  (2)65.0 m-% BaNd₂Ti₄O₁₂/35.0 m-% Nd₂Ti₂O₇  (3)

The mass percent values (m-%) derive from the volume percentage ν₁, ν₂estimated from the mixing rule upon employment of the density ρ₁=5.79g/cm3 for BaNd₂Ti₄O₁₂ and ρ₂=6.05 g/cm3 for Nd₂Ti₂O₇. The calculated TKCvalues (TKC cal.) have been obtained according to the mixing ruleTKε=Σ_(i) ν_(i) TKε_(i) (i=1, 2) upon employment of TKε₁=−111 ppm/K forBaNd₂Ti₄O₁₂ and TKε₂=217 ppm/K for Nd₂Ti₂O₇ with the assumption of anexpansion coefficient of the ceramic specimens α_(L)=8 ppm/K accordingto TKC=TKε+α_(L). The temperatures in the index of the TKC valuesindicate the interval in which the indicated TKC value was determined.

TABLE 1 Properties of ceramic specimens having the compositions (1), (2)and (3) TKC_(+25/) (Z) (S) (S) _(+125°C.) (Z) TKν_(0+25/) Spec- tan δTKC (cal.) Qν₀ _(55°C.) imen ε [ppm/K] [ppm/K] [ppm/K] [THz] [ppm/K] (1)— <1 × 10⁻³ 6.3_(−55/25°C.) 41 — — 25_(25/125°C.) (2) 62 0.2 × 10⁻³−15_(−55/25°C.) 20.2 4.2 (at 5.6 +14 6_(45/125°C.) GHz) (3) 64 0.2 ×10⁻³ −14_(−55/25°C.) 8.5 4.5 (at 5.4 +20 5_(45/125°C.) GHz)

The TKC values have been selected with a deviation toward positiveamounts in order to thus take the lowering caused by the glass fritadditive into consideration. Due to the absence of PbO, the ceramicsassure an enhanced stability with respect to reduction in the sinteringunder inert conditions, for example under a nitrogen atmosphere.

Common sinterability with the copper electrodes is achieved in that thephase mixture of the compounds BaNd₂Ti₄O₁₂ and Nd₂Ti₂O₇ produced fromthe oxide constituents at 1180° C. is additively laced with 3 through 10weight percent of the system ZnO—B₂O₃—SiO₂, preferably having thespecific composition (ZnO)_(58.5)(B₂O₃)_(31.45)(SiO₂)_(10.05), and themixture is subjected to a grinding process in an aqueous suspensionuntil an average grain size of 0.6 μm is obtained given approximatelymonomodal distribution.

After filtration and drying, the slurry obtained in this way isfurther-processed into granules upon addition of a pressing aid,disk-shaped or cylindrical green members being pressed therefrom orbeing processed into films immediately after addition of a suitableorganic binder system or, respectively, being converted into a pressablegranulate by spraying.

By applying Cu paste with silkscreening, the film is provided with anelectrode structure suitable for capacitors of a specific capacitanceand type, so that, following stacking, laminating and cutting, greenparts are obtained that can be subjected to decarbonizing and sintering.The FIGURE shows the structure of such a multi-layer capacitor.

For decarbonization, the green members are exposed to a gas stream ofpurest nitrogen (2 through 5 l/min, residual oxygen partial pressure<10⁻² Pa) in a furnace with a controlled atmosphere, between 2 and 23 gwater steam per hour being dosed thereto. Heating is first carried outto 400° C., held for two hours, subsequently brought to 680 through 750°C., whereby the complete decarbonization requires a reaction time of upto 6 hours. Subsequently, the delivery of water steam is reduced toabout 1 g/h and the sinter compression is implemented at 900 through1000° C.

Following the prescribed stoving curve of the appertaining copper paste,the outside Cu metallization occurs in a separate process step, likewiseunder purest nitrogen in the presence of water steam, in order to avoida reducing modification of the ceramic due to the binder constituentscontained in the paste.

Disk-shaped specimens S(Ø12-13 mm, thickness 0.6-0.7 mm) provided withCu electrodes prove suitable for determining the dielectric ceramicproperties.

Table 2 recites examples of disk specimens (S) of the inventive ceramicmasses and of multi-layer capacitors (K) that were obtained on the basisof the ceramic masses54.8 m-% BaNd₂Ti₄O₁₂/45.2m-% Nd₂Ti₂O₇  (1)61.4m-% BaNd₂Ti₄O₁₂/38.6m-% Nd₂Ti₂O₇  (2)65.0m-% BaNd₂Ti₄O₁₂/35.0m-% Nd₂Ti₂O₇  (3)68.0m-% BaNd₂Ti₄O₁₂/32.0m-% Nd₂Ti₂O₇  (4)with a respective additive of 6 weight percent glass frit having thecomposition (ZnO)_(58.5)(B₂O₃)_(31.45)(SiO₂)_(10.05). The specimens (S)and (K) have been produced as the result of a common sintering with theCu electrodes. 24 multi-layer capacitors (K) having a capacitance of89±1 pF were produced.

In addition to the respective sintering temperature T_(sinter) and thesintering time t_(Sinter), Table 2 indicates the relative densityδ_(rel.) in % referred to the theoretically maximally obtainable densityas the criterion for the porosity of the specimens, the loss angle tanδ, TKC and the insulation resistance RIs of the ceramic specimens.

TABLE 2 Properties of ceramic specimens (S) and (K) on the basis of theabove ceramic masses (1) . . . (4) ρ_(rel) tan δ TKC RIs SpecimenT_(Sinter)/t_(Sinter) [%] ε (1MHz) [ppm/K] [MΩ] S(1) 1000° C./1 h 46 0.4× 10⁻³ 75_(−55/+25°C.) >10⁶ 85_(+25/125°C.) S(2)  975° C./6 h 98 54 0.4× 10⁻³ 6_(−55/+25°C.) >10⁶ −5_(+25/125°C.) S(2)  950° C./6 h 94 51 0.6 ×10⁻³ 6_(−55/+25°C.) >10⁶ −4_(+25/125°C.) S(2)  930° C./6 h 97 53 0.5 ×10⁻³ 7_(−55/+25°C.) 3 × 10⁵ −2_(+25/125°C.) S(2) 1000° C./1 h 97 46 0.4× 10⁻³ −10_(−55/+25°C.) >10⁶ −10_(+25/125°C.) S(3)  975° C./6 h 96 580.6 × 10⁻³ −13_(−55/+25°C.) >10⁶ −11_(+25/125°C.) S(3)  950° C./6 h 9855 0.6 × 10⁻³ −6_(−55/+25°C.) >10⁶ −15_(+25/125°C.) S(3)  930° C./6 h 9755 0.4 × 10⁻³ −15_(−55/+25°C.) 2 × 10⁵ −36_(+25/125°C.) K(4) 1000° C./3h 0.4 × 10⁻³ 5_(+25/125°C.) 2 × 10⁷

The specimens S(2) and S(3) show that the admixture of 6% of the glassfrit to the ceramic mass already enables an adequate sinter compressionin the presence of the Cu electrodes beginning with 950° C. and that thematerial properties demanded of a COG capacitor ceramic are met.

The invention is not limited to the illustrated exemplary embodimentsbut is defined in its most general form by the patent claims.

1. A ceramic mass comprising a phase-heterogeneous ceramic with anadditive of glass frit, said phase-heterogeneous ceramic comprising mweight percent of a phase composed of BaNd₂Ti₄O₁₂ with a negativetemperature coefficient of the dielectric constant and 100-m weightpercent of a phase composed of Nd₂Ti₂O₄ with a positive temperaturecoefficient of a dielectric constant, wherein m has a value of 50<m<70;and wherein the glass frit contains ZnO, B₂O₃ and SiO₂ and has a weightamount of between 3 and 10 weight percent of the ceramic.
 2. A ceramicmass according to claim 1, wherein the glass frit comprises acomposition (ZnO)_(58.5)(B₂O₃)_(31.45)(SiO₂)_(10.05.)
 3. A capacitorcomprising a base body of ceramic, said base body, on two oppositesides, having contact layers and a plurality of electrodes engaging thecontact layers and extending into the ceramic body comb-like relative toone another, said ceramic body being a ceramic mass comprising aphase-heterogeneous ceramic and an additive of glass frit, saidphase-heterogeneous ceramic comprising m weight percent of a phasecomposed of BaNd₂Ti₄O₁₂ with a negative temperature coefficient of thedielectric constant and 100-m weight percent of a phase composed ofNd₂Ti₂O₄ with a positive temperature coefficient of a dielectricconstant, wherein m has a value of 50<m<70; and wherein the glass fritcontains ZnO, B₂O₃ and SiO₂ and has a weight amount of between 3 and 10weight percent of the ceramic.
 4. A capacitor according to claim 3,wherein the electrodes are composed of copper and the ceramic body is asintered ceramic mass sintered with the copper electrode.
 5. A capacitoraccording to claim 4, wherein the composition of the ceramic provides atemperature coefficient capacitance for the capacitor in a temperatureinterval between −55° C. and 125° C. and less than 30 ppm/Kelvin.
 6. Acapacitor according to claim 3, wherein the composition of the ceramicprovides a temperature coefficient capacitance for the capacitor in atemperature interval between −55° C. and 125° C. and less than 30ppm/Kelvin.
 7. A capacitor according to claim 3, wherein the glass fritcomprises a composition (ZnO)_(58.5)(B₂O₃)_(31.45)(SiO₂)_(10.05.)
 8. Acapacitor according to claim 7, wherein the composition of the ceramicprovides a temperature coefficient capacitance for the capacitor in atemperature interval between −55° C. and 125° C. and less than 30ppm/Kelvin.
 9. A capacitor according to claim 7, wherein the electrodesare composed of copper, the ceramic mass is a sintered mass sinteredtogether with the copper electrodes.
 10. A capacitor according to claim9, wherein the composition of the ceramic provides a temperaturecoefficient capacitance for the capacitor in a temperature intervalbetween −55° C. and 125° C. and less than 30 ppm/Kelvin.